<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }

      .content {
        overflow: hidden;
        width: 1000px;
        height: 3000px;
        background-color: pink;
        margin: 0 auto;
      }

      .backtop {
        display: none;
        width: 50px;
        left: 50%;
        margin: 0 0 0 505px;
        position: fixed;
        bottom: 60px;
        z-index: 100;
      }

      .backtop a {
        height: 50px;
        width: 50px;
        background: url(./images/bg2.png) 0 -600px no-repeat;
        opacity: 0.35;
        overflow: hidden;
        display: block;
        text-indent: -999em;
        cursor: pointer;
      }

      .header {
        position: fixed;
        top: 0;
        left: 0;
        width: 100%;
        height: 80px;
        background-color: purple;
        text-align: center;
        color: #fff;
        line-height: 80px;
        font-size: 30px;
        transition: all 0.3s;
      }

      .sk {
        width: 300px;
        height: 300px;
        background-color: skyblue;
        margin-top: 500px;
      }
    </style>
  </head>

  <body>
    <div class="header">我是顶部导航栏</div>
    <div class="content">
      <div class="sk">秒杀模块</div>
    </div>
    <div class="backtop">
      <img src="./images/close2.png" alt="" />
      <a href="javascript:;"></a>
    </div>
    <script>
      // 获取元素
      const sk = document.querySelector(".sk");
      const header = document.querySelector(".header");
      // 监听滚动事件
      window.addEventListener("scroll", function () {
        // 页面滚动到秒杀模块就改变头部top值
        // 页面被卷去的头部 >= 秒杀模块的位置offsetTop
        const n = document.documentElement.scrollTop;
        // if (n >= sk.offsetTop) {
        //   header.style.top = 0;
        // } else {
        //   header.style.top = "-80px";
        // }

        // 优化三元运算符
        header.style.top = n >= sk.offsetTop ? 0 : "-80px";
      });
    </script>
  </body>
</html>
